During the ongoing All Sky Automated Survey for SuperNovae (ASAS-SN, Shappee et al. 2014), using data from the quadruple 14-cm "Cassius" telescope in Cerro Tololo, Chile, we discovered a new supernova in the galaxy NGC 2996.

ASASSN-17df (AT 2017bos) was discovered in images obtained on UT 2017-03-01.08 at V~16.4 mag. We also detect the object in images obtained on UT 2017-03-05.13 (V~16.3) and UT 2017-02-22.24 (V~17.1). We do not detect (V>18.2) the object in images taken on UT 2017-02-19.41 and before. An image obtained on UT 2017-03-01 by G. Stone confirms the discovery of the transient. This figure shows the archival Pan-STARRS (Chambers et al. 2016, arXiv:1612.05560) g-band image of the host (left) and the G. Stone confirmation image (right). The red circle has a radius of 5" and is centered on the position of the transient in the G. Stone image.

The position of ASASSN-17df is approximately 0.1" North and 0.1" East from the center of the galaxy NGC 2996 (z=0.029270, d=128 Mpc, via NED), giving an absolute V-band magnitude of approximately -19.3 (m-M=35.53, A_V=0.129). Properties of the new source and photometry are summarized in the tables below:

We obtained a optical spectrum of ASASSN-17df on UT 2017 Mar 2 with FAST mounted on the FLWO 1.5m telescope and with the SAAO 1.9m telescope. The spectra show features consistent with a Type Ia SN around maximum light on top of strong host galaxy contamination. We measure Si II 6355 Angs. expansion velocity of 9800 km/s from the minimum of the P-Cygni absorption trough.

While we are participating in the TNS system to minimize potential confusion, ASAS-SN will continue using ASASSN-17xx transient names as our primary nomenclature (including supernovae, but also other classes of transients), and we encourage others to do the same. We prefer merging the names as ASASSN-17xx (AT 2017xyz) to preserve, rather than anonymize, the origin of the transient.

We thank Las Cumbres Observatory and its staff for their continued support of ASAS-SN. ASAS-SN is funded in part by the Gordon and Betty Moore Foundation through grant GBMF5490 to the Ohio State University, NSF grant AST-1515927, the Center for Cosmology and AstroParticle Physics (CCAPP) at OSU, the Chinese Academy of Sciences South America Center for Astronomy (CASSACA), and the Mt. Cuba Astronomical Foundation. For more information about the ASAS-SN project, see the ASAS-SN Homepage and the list of all ASAS-SN transients.